1. Field of the Invention
The present invention relates to an automatic transmission for a vehicle and more particularly to an automatic transmission provided with a belt drive continuously-variable speed transmission mechanism.
2. Description of the Prior Art
A belt drive continuously-variable speed transmission mechanism is employed in combination with a torque converter or a fluid coupling, and a changeover mechanism including a forward drive and a reverse drive in an automatic transmission for a vehicle.
The belt drive continuously-variable speed transmission includes generally an input shaft, an input variable-pitch pulley having a fixed pulley flange fixed concentrically to the input shaft and a movable pulley flange mounted concentrically with and axially slidably on the input shaft, an output shaft disposed in parallel with the input shaft, an output variable-pitch pulley having a fixed pulley flange fixed concentrically to the output shaft and a movable pulley flange mounted concentrically with and axially slidably on the output shaft, an endless belt interconnecting the input and the output variable-pitch pulleys and hydraulic servomotors provided for the input and the output variable-pitch pulleys respectively for pressing the corresponding movable pulley flanges toward the corresponding movable pulley flanges, and a hydraulic control circuit including a pressure generating source, a regulator valve to control the fluid pressure of the pressure generating source and adapted to generate a line pressure corresponding to pressure signals fed thereto, a throttle pressure valve to control the line pressure and adapted to generate a pressure corresponding to a throttle opening of the internal-combustion engine and to feed the pressure to the regulator valve as a throttle pressure signal, a reduction ratio detecting valve to control the line pressure and adapted to generate a pressure corresponding to the axial displacement of the movable pulley flange of the output variable-pitch pulley mounted on the output shaft and to feed the pressure to the regulator valve as a reduction ratio pressure signal, means to supply the working fluid at the line pressure to the hydraulic servomotor formed within the output variable-pitch pulley, and a reduction ratio control mechanism including a reduction ratio control valve having an axially slidable spool and two pressure chambers formed in the valve body thereof at both axial ends of the spool respectively and adapted to supply the line pressure selectively to the hydraulic servomotor of the input variable-pitch pulley through the movement of the spool, fluid passages to conduct the working fluid at the line pressure to the pressure chambers and solenoid valves to regulate the respective pressures in the two pressure chambers respectively.
Since those component solenoid valves of the reduction ratio control mechanism included in the hydraulic control circuit function to maintain the line pressure in the two pressure chambers of the reduction ratio control valve or to drain the same pressure chamber, capacity and durability which are sufficient to withstand the line pressure are required of the solenoid valves.